Integrated semiconductor devices are typically constructed en masse on a wafer of silicon or gallium arsenide. Each device generally takes the form of an integrated circuit (IC) die, which is attached to a leadframe with gold wires. The die and leadframe are then encapsulated in a plastic or ceramic package, which is then recognizable as an IC "chip". IC chips come in a variety of forms such as dynamic random access memory (DRAM) chips, static random access memory (SRAM) chips, read only memory (ROM) chips, gate arrays, and so forth. The chips are interconnected in myriad combinations on printed circuit boards by any number of techniques, such as socketing and soldering.
Interconnections among chips arrayed on printed circuit boards are typically made by conductive traces formed by photolithography and etching processes. Semiconductor circuit devices, including DRAMs, SRAMs and gate arrays are essentially switching devices. As the output drivers within those chips create intermittent current flow on associated conductive traces, the traces behave as inductors, creating voltage surges which have the potential for creating logic errors. Other logicdamaging transient voltages, caused by voltage fluctuations at the power line and the interaction of other circuit components in the system, may also be present.
In order to render innocuous the transient voltages which regularly appear in logic circuits, decoupling capacitors are commonly used as lowfrequency bypass filters.
One circuit-board-mounted semiconductor chip array that is of particular interest is the Single Inline Memory Module (SIMM). A SIMM is a highly space-efficient memory expansion board having no onboard address circuitry. It is designed to be plugged directly into the address, data and powersupply buses of a computer so that cells within the chips comprising its randomly-addressable memory can be addressed directly by the computer's CPU rather than by a bank-switching technique commonly used for larger memory expansion boards. The SIMM's memory cells are perceived by the computer's CPU as being no different than memory cells within chips connected directly to the computer's motherboard. SIMMs are typically furnished in multiple-byte or multiple-word configurations. That is to say that for any eightbit byte or sixteen-bit word of information stored on a SIMM, each of the component bits will be found on a separate chip and will be individually addressable by column and row. One edge of a SIMM module is a card-edge connector, which plugs into a socket on the computer which is directly connected to the computer buses required for powering and addressing the memory on the SIMM.
FIG. 1 is a top plan view of a typical SIMM. This particular SIMM comprises a printed circuit board 11 on which nine 256K-bit DRAM chips D1 through D9 are surface mounted. Each of the DRAM chips has eighteen J-mount connector pins 13 which have been soldered, using infrared energy, to eighteen chip connector pads 21 (shown in FIG. 2) on the SIMM beneath each chip. Circuit board 11 has a card-edge connector 15 comprised of a series of thirty rectangular terminal pads 17 on each side of the board. Thirty terminal pads 17 are visible in FIG. 1. Card-edge connector 15 plugs into a socket on the computer motherboard, providing parallel connection to the computer's address, data, power and ground buses.
FIG. 2 is a top plan view of the SIMM of FIG. 1 with DRAM chips D1 through D9 removed, completely exposing the nine sets of eighteen chip connector pads 21, as well as eight of the nine surface-mount decoupling capacitors C1 through C9. Capacitor C9 has been removed in order to expose an upper capacitor mounting pad 23A and a lower capacitor mounting pad 23B. On this particular SIMM, each upper capacitor mounting pad 23A is connected to both the Vcc bus and, by means of upper conductive trace 25, to chip connector pad 21C, which is the Vcc input to the DRAM chip. Likewise, each lower capacitor mounting pad 23B is connected to both the ground-plane bus and, by means of lower conductive trace 27, to chip connector pad 21L, which is the DRAM chip's connection to ground. The Vcc bus and the ground-plane bus on circuit board 11 are not visible in the plan views of FIGS. 1 and 2, since those particular bus traces are located between two of the board's six layers.
From the foregoing description of the SIMM depicted in FIGS. 1 and 2, it will be noted that all nine of the SIMM's decoupling capacitors are connected in parallel between the Vcc bus and the ground plane bus. As long as the dielectric of each of the nine capacitors is intact, the module is functional. However, a short in any one of the nine capacitors will result in the Vcc bus becoming shorted to the ground-plane bus, whereupon the SIMM will begin to draw an inordinate amount of current which will invariably result in its destruction.
Decoupling capacitors of the surface-mount type are particularly susceptible to shorting, since they have no leads to thermally isolate them as they are soldered to a circuit board with infrared energy, at temperatures of up to 700 degrees Fahrenheit. Even if a surface-mount capacitor survives the mechanical shock generated by soldering process, it is still vulnerable to other types of mechanical stress that would not affect a lead-mounted capacitor. For example, by simply bending a SIMM having surface-mounted capacitors, the capacitors may be compromised. And, even if a SIMM passes testing following manufacture (an indication that none of the decoupling capacitors are initially shorted), it may have a relatively high failure rate when placed in use. SIMMs, of the type shown in FIG. 1, may have an average failure rate traceable to shorted decoupling capacitors as high as one per thousand.
Single inline packages (SIPs) are similar in design to SIMMs, except that instead of having a card edge-type connector, SIMMs have pins which are either socketably or solderably mounted on a bus. The problems associated with the decoupling capacitor system of SIMMs also apply to SIPs.